We seek to characterize and analyze the visual pigments in single receptor cells of vertebrates and invertebrates (a) to understand their role in visual perception and color discrimination; (b) better to understand the chemical and configurational changes that take place in the pigments subsequent to illumination; (c) to detect and measure changes in free ions and metabolic intermediates in receptors in darkness and light; (d) to relate pigment changes to electrical activity in photoreceptors; (e) to maintain a core facility for single receptor spectrophotometry which may be used by other scientists having suitable problems. We plan to concentrate upon the measurement of cone pigments since these have been much less studied than those of rods due to the difficulty of extracting them and isolating various types in vitro. Thus far the cone pigments of very few common mammals and fishes have been studied. Our spectrophotometric studies will be correlated with histological studies of the receptor mosaic using methods such as Nitro blue Tetrazolium staining, which can be used to distinguish between different spectral classes of cones. Chemical studies will be done on the rods of amphibia, because of their very large size, using the technique of outer segment isolation in a heavy oil suspension recently developed in our laboratory. Studies on putative excitation transmitters using radioactive tracer techniques, already underway, will be continued. All these studies with simultaneous electrophysiological measurements will also be done on the ventral photoreceptor cells of Limulus.